Abstract: Herein, we report a facile low-temperature (80˚C) synthesis route for colloidal cobalt (Co) nanoparticles by Co2+ reduction using hydrazine (N2H4) in a basic solution of ethylene glycol (EG). The colloidal particles were spherical and monodispersed, with mean diameter ranging from 2–7 nm increasing with the Co chloride concentration. However, precipitation of the nanoparticles resulted in weakly agglomerated spherical Co structures of submicron size. X-ray diffraction (XRD) of the precipitated powder revealed hexagonal close-packed-Co (hcp-Co). But since the major peaks of hcp and face-centered cubic-Co (fcc-Co) are overlapping, the presence of fcc-Co cannot be ruled out. Besides precursor concentration, it was found that pH, reaction temperature, and molar ratio of N2H4 to Co2+ also influence the reduction rate. An alkaline medium, elevated temperature (80˚C) and high molar ratio of N2H4 to Co2+ promoted the formation of colloidal Co nanoparticles. Nonetheless, when the m...